When choosing the best digital multimeter (DMM) with the right amount of horsepower for your application, it is important to get the most efficient instrument and the best return on your investment. Just like you would not use a yardstick to measure distance between atoms or a canon to eradicate a pesky fly, you would not use a system-based DMM to make occasional voltage measurements.
Other platforms are available, including VXI, PC-based, and benchtop. But which is best for you? The selection often is based on your accuracy and test-automation requirements.
System/Benchtop DMM
The traditional benchtop DMM is designed primarily for stand-alone manual operation and has buttons with a large display to aid the measurement process, said Tom Wultich, VXI product marketing manager at Tektronix. With resolution as high as 8-1/2 digits, the benchtop DMM is the choice for demanding tasks such as those performed in calibration labs.
A secondary application for benchtop DMMs is simple automated testing, usually from the IEEE 488 port to a PC, continued Mr. Wultich. The IEEE 488 port primarily is a spigot for moving data to a PC. But aside from the availability of a few plug-in scanning channel modules, the benchtop is not designed to operate with other instruments. It is a popular choice if you use the DMM for manual operation and occasionally need to run an automated test.
The benchtop DMM fits well in the small- to medium-size application requiring medium to very high measurement accuracy, said Tom Hayden at Keithley Instruments. When only a few tests are required on a small number of devices, a single DMM equipped with a scanner card can automate the testing process.
Including GPIB and RS-232 interfaces as standard features in many moderately priced DMMs has blurred the distinction between benchtop and system DMMs, said Mark Bailey, product manager at Hewlett-Packard. Full functionality, 6-1/2- digit resolution, excellent accuracy, and both GPIB and RS-232 interfaces now are available for less than $1,000. This is good news for the typical users of the benchtop DMM, including R&D engineers, technicians, production test and repair personnel in the manufacturing environment, and students.
Benchtop DMMs with a GPIB interface are used in production test systems, primarily if you do not want to migrate to VXI, or your systems are too small for VXI, said James Kimery at National Instruments. Typically, GPIB DMMs are not software controlled because they perform single-shot measurements on the bench. Often they do not connect easily to standard switches, but they are particularly useful for applications needing precise measurements.
Many system DMMs are purchased as bench-type instruments but with an eye toward incorporating them into a system or at least a connection to a PC for data control or instrument control via the PC, said HP’s Mr. Bailey. They are designed for higher throughput and accuracy than the benchtop with 7-1/2- to 8-1/2-digit resolution and up to 100,000 readings/s throughput. Applications range from small production test to large ATE.
VXI-Based DMMs
The VXI-based test system typically is used for large, specialized applications requiring medium- to high-precision measurements, said Keithley’s Mr. Hayden. The test system contains different instruments that perform many measurements per test or accesses test points per channel that number in the thousands or tens of thousands.
VXI-based DMMs are suited for automated testing because the VXI backplane can move data faster than the IEEE 488 interface, said Mr. Wultich of Tektronix. Additionally, VXI has triggering capabilities that allow the DMM to operate with other instruments in an automated test environment. The VXI-based test system includes triggering scanners, switching matrices, or triggering for other instruments such as counters and function generators.
One of the main benefits of using a VXI-based instrument is its advanced switching technology, said Mr. Kimery. In a relatively small space, hundreds of test points can be measured using a single VXI DMM. The combination of a VXI DMM and a switch gives a test system a low-cost solution to measure a wide variety of signals without expensive dedicated hardware.
Joining VXI DMMs and switches also provides a flexible interface to several UUTs, said Mr. Kimery. One VXI system can test many UUTs by simply changing the interface adapter and the software. Because the DMM measures a variety of signals, it can handle a wide range of applications.
PC-Based DMMs
PC-based DMMs typically are used in medium-sized applications where small size, ease of integration and programming simplicity are desirable, said Keithley’s Mr. Hayden. Also, they are appropriate for portable applications, field service, and low-cost random-signal measurement applications.
PC-based instruments also take advantage of Windows features including Internet access and word-processing functions, noted Mr. Kimery of NI. The PC also offers high-resolution displays and expandable memory for ease of use and to keep pace with more complex tasks.
The PC-based measurement system uses one system environment for all functions, said John Hughes, director of sales and marketing at Signametrics. It has one power supply, a one-person-to-machine interface, and one interconnecting bus. Signals readily get from the device-under-test to the test-instrument via cables that connect directly to the test instrument terminals on the back of the PC.
Custom programs also can be written for the PC-based DMM to perform data analysis, data logging, and data plotting, said Kevin Kline, sales engineer at Quatech. Programming in the Windows environment is simple and can be used to write customized software for DMM applications, and control measurements and sampling rates.
Software Issues
Selecting the best DMM platform is not just a hardware issue—you also must look at the software. The availability and the quality of the driver are the most significant issues for test software, said Mr. Wultich at Tektronix. Additionally, the operating system and the application development environment must be considered.
In the past, the third party providers of application software often wrote the drivers for instruments. It helped them sell their software easily. The more drivers the better was the rule. Unfortunately, these providers sometimes did not have the actual instrument to verify the operation of the driver, and the driver needed some tweaking to operate properly.
For automated tests, choose a DMM with an instrument driver, counseled NI’s Mr. Kimery. Without an instrument driver, you must write much of the software from scratch, incurring added development costs.
Picking the optimal hardware for an application or reusing old hardware often requires that driver modules from several providers communicate with each other, said Mr. Hughes of Signametrics. This is risky business. A better route is to choose the system elements with a view to keeping the number of software third party providers to an absolute minimum.
Familiarity with the software for your platform is another issue to resolve, pointed out Mr. Hayden at Keithley Instruments. For example, using a PC-based software package for benchtop instruments and an IEEE 488 bus with the standard commands for programmable instruments (SCPI) set is simple because it does not require knowledge of a complex programming language. PC-based systems can be developed using graphical programming techniques that minimize writing source code.
Today, platforms such as VXI use VXIplug&play guidelines that require instrument drivers be supplied by the manufacturer, said Mr. Wultich. These are more reliable because they are developed by engineers who are familiar with all the nuances of the instrument.
VXIplug&play DMMs are supplied with instrument drivers for C or LabVIEW written in source code, said NI’s Mr. Kimery. They also have executable software front panels to let you immediately exercise the instrument without programming. VXI instruments that are not plug-and-play compatible may not have the drivers or a software front panel.
Another advancement for VXI-based instruments is the virtual instrument software architecture (VISA), said Mr. Kimery. With VISA, an instrument vendor can write a driver once, and it will work with all VXIplug&play-compliant controllers, even the embedded, MXI, and GPIB VXI products.
VXIplug&play, however, makes no pledges that replacing one vendor’s DMM with another will work without some software rework, said Mr. Wultich at Tektronix. That would require instruments with identical functions such as voltage ranges. You also may not be able to port your code to a PC-based instrument or vice versa without adjusting the software.
GPIB-based DMMs usually operate without software. So if you use it for automated test, make sure the instrument has a driver for your specific language, operating system, and application environment, said NI’s Mr. Kimery. Also, you must receive the instrument driver in source code so you can tailor it to your application.
Like VXI, the PC-based DMM is useless unless the driver software is distributed with the instrument, said Mr. Kimery. Additionally, PC DMMs should include turnkey, ready-to-run software so you can quickly turn your PC into an instrument.
Currently, instrument driver software is not 100% interchangeable. The SCPI set comes close because it is portable among the different platforms. The SCPI set facilitates a message-based interface to standard instruments. DMMs that comply and implement a SCPI parser use the same set of commands. This provides a level of interchangeability below the instrument driver, but only for message-based VXI devices.
DMMs have dramatically improved in resolution and accuracy in the last decade, and now offer more focused solutions and more platforms. In addition, testing is more intuitive with the advent of graphical programming and simplified with the arrival of driver standards.
The right DMM is available for you, whether it is a benchtop, a VXI-based, a GPIB, or a PC-based platform. Before you buy your next meter, check the chart in this article to compare the features, platform, and price.
Copyright 1997 Nelson Publishing Inc.
August 1997